This invention relates to process controllers, and is particularly applicable to process controllers for use in diameter control of filaments, such as for instance in the production of glass optical fibre from preform, or of plastics coated wire or optical fibre.
In a drawing down process, such as the production of glass optical fibre from optical fibre preform, uniformity of diameter of the drawn fibre is provided by monitoring its diameter to provide a control signal which is fed to a process controller which controls a servo-motor that either regulates the rate of the feed of the preform into a drawing zone or regulates the rate of take-up of the drawn filament from the drawing zone. In the case of optical fibre it is generally preferred to have the servo-motor regulating the take-up speed. In the corresponding case of the production of a plastics coated optical fibre or wire, the diameter of the plastics coated fibre or wire is monitored, and the process controller controls a servo-motor that either regulates the feed of plastics material through an extruder, or regulates the line speed of the fibre or wire through the extruder.
One way of measuring the diameter of a filament, such as a bare optical fibre or a plastics coated fibre or wire, is to measure the optical attenuation it produces in a beam of light that is partially intercepted by the filament. Normally this measurement is made by a comparison technique in which a beam splitter is used to divide a light beam into two components, one of which is partially obstructed by the filament, while the other passes through an attenuator whose optical attenuation is adjustable.
The intensities of the two beams are then compared by chopping them and feeding them both to the same photodetector to provide a difference signal whose amplitude is approximately linearly proportional to the deviation of the filament diameter from a predetermined reference value set by the adjustable optical attenuator. This difference signal is rectified and fed to a process controller which provides an output for controlling the speed of the servo-motor.
For certain applications in wire production, it has been found satisfactory to use a type of process controller which operates by sampling the analog signal from the diameter sensing unit, and using this to increment (or decrement as appropriate) a motorized potentiometer that controls the servo-motor speed. The increment size remains constant after being preset, but the sampling repetition rate is a function of the magnitude of the deviation signal. To set up such an apparatus it is necessary to adjust the increment magnitude to a value small enough for a single increment not to produce too large a shift. However, although a reduction in the size of an increment tightens the control on the size, it also has the effect of slowing down the response of the system to perturbations. In many wire production applications, the perturbations are small enough, having regard to the effects they produce, for a relatively slow response to be acceptable. However, we have found that in the drawing of glass optical fibre from fibre preform, the drawing conditions are so sensitive to small changes that a process controller that was satisfactory for diameter control in wire production gave inadequate diameter control for glass fibre drawing. Thus for instance it was found that in drawing 100 micron diameter fibre from 6mm preform it was possible to hold the fibre diameter to about .+-. 3 microns over a region in which the preform diameter was substantially uniform, but, in drawing from a region where the diameter of the preform rose and fell by 5.7% over a distance of about 11/2cm, the fibre diameter fluctuated in an oscillatory manner by about .+-. 25%. Clearly, a tighter control and a faster non-oscillatory response is desirable.